How to change angles of a gameobject along the curve (Unity2D)

Question

I am trying to move a gameobject along the curve. In addition to it I want to change the angle of that gameobject accordingly.

Following is my code. But its not working properly:

public class CharacterController : MonoBehaviour {

Vector3 prevPos, currentPos;
Vector3 perpPos;

// Use this for initialization
void Start () {
    Debug.Log ("Character Controller start method");
    prevPos = gameObject.transform.position;
    currentPos = gameObject.transform.position;
}

// Update is called once per frame
void Update () {
    currentPos = this.transform.position;
    perpPos = new Vector3 (currentPos.x, prevPos.y, 0.0f);
    //Debug.Log ("perpPos" + perpPos);
    float perpDist = Vector3.Distance (perpPos, currentPos);
    float baseDist = Vector3.Distance (prevPos, perpPos);
    //Debug.Log ("perpDist " + perpDist);
    //Debug.Log ("baseDist " + baseDist);
    float angle = Mathf.Tan (perpDist / baseDist);
    angle = angle * Mathf.Rad2Deg;
    Debug.Log ("angle " + angle);
    gameObject.transform.rotation = Quaternion.Euler (0,0, angle);
    this.transform.eulerAngles = new Vector3 (0, 0, angle);
    //Debug.Log ("angle " + angle);

    //Quaternion rot=new Quaternion();
    //rot.eulerAngles = new Vector3(0, 0, angle);
    //transform.rotation=rot;

    prevPos = currentPos;

}

}

Here I am getting the previous position and current position of the gameobject. Calculating the 3rd point so that I can get the perpendicular and base of the 2 individual vectors. From there I am calculating the angle using tan(Theta) = perp/base formula. But this does not seem to work smoothly.

If anyone can help?